1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, particularly to an electrophotographic photosensitive member containing an azo pigment having a specified structure as a charge-generating substance.
2. Related Background Art
The organic photosensitive members are practically used which are mainly composed of an electrophotosensitive polymer, as an organic photosensitive substance, typified by poly-N-vinylcarbazole, and charge transfer complexes thereof with a Lewis acid such as 2,4,7-trinitro-9-fluorenone. Those photosensitive members are not necessarily satisfactory in their sensitivity and durability.
On the other hand, a function-separate type of electrophotographic photosensitive member in which a charge-generating function and a charge-transporting function are respectively imparted to separate substances has achieved a remarkable improvement in sensitivity and durability which are deficient in conventional organic photosensitive members. Such function-separate type photosensitive members are advantageous in that the charge-generating substance and the charge-transporting substance can respectively be selected from a variety of substances, thus facilitating the preparation of an electrophotographic photosensitive member having any desired characterisitc. Since electrophotographic photosensitive members are being used not only for copying machines but also for laser beam printers, LED printers, etc., the function-separate type photosensitive members are suitable for providing a spectral sensitivity region in accordance with the wavelength of the light source employed.
A variety of charge-generating substances are known such as azo pigments, phthalocyanine pigments, polycyclic quinone pigments, cyanine pigments, squaric acid pigments, and pyrylium salt pigments. Among them, various kinds of azo dyes are reported because of their high light-fastness, high-charge-generating ability, and ease of synthesis.
Electrophotographic photosensitive members, for example, are known which contain an electroconductive azo pigment as a charge-generating substance in a photosensitive layer comprising functionally separated layers of a charge-generating layer and a charge-transporting layer as disclosed in U.S. Pat. Nos. 4,123,270, 4,247,614, 4,251,613, 4,251,614, 4,256,821, 4,260,672, 4,268,596, 4,278,747, and 4,293,628.
Azo pigments for the charge-generating substance are required to be stable to heat and light, to have a high charge-generating ability independently of temperature, not to change characteristics on repeated use, to have an effective spectral sensitivity range corresponding to the object of application, not to restrict the selection of the charge-transporting substance, and so on. Only limited numbers of azo pigments have been practically used because the above requirements have to be satisfied to a high degree.